U.S. patent number 4,465,902 [Application Number 06/433,344] was granted by the patent office on 1984-08-14 for digital space phone system.
This patent grant is currently assigned to Zenith Electronics Corporation. Invention is credited to Thomas J. Zato.
United States Patent |
4,465,902 |
Zato |
August 14, 1984 |
Digital space phone system
Abstract
A television receiver having two-way telephone capability
includes a digitized talk channel and a digitized listen channel
each having a gain-controlled amplifier selectively controlled by a
switch controlled generator driven by the output of a subtraction
circuit. Multipliers are connected to the inputs of the subtraction
circuit and in turn receive respective inputs from their associated
channels and from the other channels for offsetting cross coupled
signals due to the telephone interface.
Inventors: |
Zato; Thomas J. (Palatine,
IL) |
Assignee: |
Zenith Electronics Corporation
(Glenview, IL)
|
Family
ID: |
23719829 |
Appl.
No.: |
06/433,344 |
Filed: |
October 8, 1982 |
Current U.S.
Class: |
348/552; 379/339;
379/345; 455/552.1; 455/570 |
Current CPC
Class: |
H04M
9/08 (20130101) |
Current International
Class: |
H04M
9/08 (20060101); H04M 011/00 () |
Field of
Search: |
;179/2R,2TV,81B,1L,84T
;358/85 ;381/110,107,108,57,94 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Rubinson; Gene Z.
Assistant Examiner: Brady; W. J.
Claims
What is claimed is:
1. A television receiver having two-way telephone communication
capability comprising:
a talk channel and a listen channel coupled to a telephone line,
said telephone line introducing substantial cross coupling between
said channels;
an A/D converter in each said channel for digitizing signals in
said channels;
a gain controlled amplifier in each said channel; and
channel control means for selectively controlling the gains of said
channel as a function of the difference in signal levels in said
channels and for offsetting cross coupled signals in said channels,
said channel control means including;
differential means comparing the signal levels in the channels and
developing an output as a function of the comparison;
a switch control generator for adjusting the gains of the channels
in accordance with the output of the differential means; and
a variable gain amplifier, associated with each channel and having
an output coupled to a respective input of the differential means
and two inputs, one input being responsive to the signal level in
the talk channel and another input being responsive to the signal
level in the listen channel.
2. A television receiver as set forth in claim 1 wherein said
switch control generator has a pair of outputs, each of which is
connected to the gain-controlled amplifier in its associated
channel and to the input of the variable gain amplifier in the
other channel.
3. A television receiver as set forth in claim 2 wherein the signal
level fed to the other channel is equal to A and the signal level
fed to the gain-controlled amplifier in said associated channel is
equal to (1-A).
4. A television receiver as set forth in claim 3 wherein the
sensitivity of said talk channel is less than the sensitivity of
said listen channel.
5. A television receiver as set forth in claim 4 further
including:
noise cancelling means in said talk channel, the input to said
gain-controlled amplifier in said talk channel being taken from the
input of said noise cancelling means and the input to its
associated variable gain amplifier in said channel control means
being taken from the output of said noise cancelling means.
6. A television receiver as set forth in claim 5 wherein said noise
cancelling means has a long time constant to compensate for
relatively constant background noise signals in said talk
channel.
7. A television receiver having two-way telephone communication
capability comprising:
a talk channel having an output circuit, including a microphone
responsive to sound adjacent to the television receiver, coupled to
a telephone line;
a listen channel having an output circuit coupled to said telephone
line, and being connectable to the television audio system, said
telephone line introducing substantial cross coupling between said
channels;
A/D converter means for digitizing output analog signals from said
microphone and from said telephone line;
a gain-controlled amplifier and a D/A converter in the output
circuits of each said talk channel and said listen channel; and
channel control means selectively controlling the gains of said
gain-controlled amplifiers as a function of the digitized signal in
said channels and for offsetting cross coupled signals in said
channels, said channel control means including:
differential means, having a pair of inputs each of which is
connected to receiver signals in said talk channel and in said
listen channel;
a switch control generator for selectively adjusting the gains of
said gain-controlled amplifiers in accordance with the output of
said differential means; and
a variable gain amplifier associated with each channel, and having
an output coupled to the input of said differential means and two
inputs, one input being responsive to signal levels in the talk
channel and the other input being responsive to signal levels in
the listen channel.
8. A television receiver as set forth in claim 7 further
including:
ring detector means responsive to a ringing signal from said
telephone line;
oscillator means coupled to said ring detector means for generating
an audible signal in response thereto; and
means supplying said audible signal to said audio system in said
television receiver.
9. A television receiver as set forth in claim 8 further
including:
means for disabling said microphone for precluding sounds adjacent
to said television receiver from being coupled into said talk
channel.
10. A television receiver as set forth in claim 9 wherein said talk
channel, said listen channel and said channel control means are
microprocessor based.
Description
BACKGROUND OF THE INVENTION
This invention relates to improvements in television receivers
having two-way telephone communication capability.
Television receivers having two-way telephone communication
capability are known in the art. U.S. Pat. No. 4,338,492 issued
July 6, 1982 to Paul A. Snopko and assigned to Zenith Radio
Corporation describes a commercially available system that enables
a TV viewer to answer or initiate a telephone call through an
operating television receiver. The receiver is equipped with a
remotely operated switch enabling the receiver to be switched to a
telephone mode. The telephone mode switching function is an added
feature on a conventional hand-held wireless remote control unit
for selecting and adjusting various television receiver
functions.
A major limitation in such telephone arrangements results from the
configuration of the telephone line, which is essentially two wires
and an induction coil. Any system for supplying signals to the
induction coil and for receiving signals from the induction coil
will inherently experience a large amount of cross coupling. Prior
art systems, including the Zenith system above, had to some degree
succeeded in solving the problem of cross coupling, generally by
including bridge circuit input/output arrangements and various
controls for "tweaking" the telephone interface circuitry to
particular line characteristics. Unfortunately, telephone lines
differ in impedance and noise characteristics. While for any given
set of line characteristics the interface circuit and amplifiers
may be optimally adjusted to minimize cross talk, less than optimum
performance will be obtained for different sets of line
characteristics.
In a two-way telephone communication system through a television
receiver the talk and listen channels are selectively operable,
that is only one channel is operable at a time. When the talk
channel is in use, the listen channel is disabled and vice versa. A
channel switching arrangement controls channel switching with the
strongest signal being the determinant. Difficulties with channel
lock-out (a condition where the "non-operating channel" signal
cannot overcome the "operating channel" signal) and too rapid
switching of channels led to incorporation of compromise time
constant networks to permit the non-operating channel to obtain
control a predetermined time after the operating channel was no
longer receiving a signal.
Another major problem is cross talk caused by a signal in one
channel feeding into the other channel which also results in rapid
switching and generally unstable conditions. To help counteract
this problem a signal threshold is imposed on the disabled channel,
requiring the signal in the non-operating channel to have a minimum
amplitude to cause channel switching.
Difficulties are still encountered despite the use of circuits with
time constants and thresholds. For example, large noises may cause
channel switching and lock-out the other channel for a substantial
time depending on the time constant of the circuit. In practice, a
"listener" might cough loudly and switch channels, thus cutting off
the "talker". The system would stay switched, keeping the talker
cut off for a time period determined by the channel time constant
and the signal level of the talker. Thus, conversation may be
arbitrarily interrupted for a varying time period, resulting in not
only unsatisfactory communication,, but also very unnatural
communication.
Similar difficulties arise because of background noise, especially
at the microphone end. Often the background noise level is
sufficiently high to make it extremely difficult for the listening
channel to become operative and also make it very annoying for the
person at the other end of the telephone line to listen to the
conversation. A mediocre or poor quality telephone connection can
also to impose serious conversational difficulties, because of weak
or noisy signals, in the listen channel. (To simplify terminology,
the terms "listen channel" and "talk channel" are arbitrarily
referenced to the television receiver location).
The listen channel is therefore preferably provided with greater
sensitivity than the talk channel, the characteristics of which are
much more controllable. However, since the signal level in the
listen channel may not be representative of the signal level
delivered from the speaker of the television receiver, optimally
establishing the sensitivities of the talk and listen channels
becomes extremely difficult. This, of course, is because the
television speaker volume is primarily under the control of the
viewer and only incidentally under the control of the other party
on the telephone line.
Despite the various techniques mentioned above and the use of
thresholds in an attempt to compensate for cross talk inherent in a
telephone line, prior art systems often fall far short of desired
performance levels.
OBJECTS OF THE INVENTION
The principal object of this invention is to provide a television
receiver having two-way telephone communication capability which
obviates many of the problems of the prior art.
A further object of this invention is to provide a television
receiver having two-way telephone communication capability that
compensates for telephone line cross coupling.
SUMMARY OF THE INVENTION
In accordance with the invention a television receiver having
two-way telephone communication capability includes a talk channel
and a listen channel coupled to a telephone line which introduces
substantial cross coupling between the channels, channel control
means selectively controlling the gains of the channels as a
function of the signals in the channels and for offsetting cross
coupled signals in the channels.
BRIEF DESCRIPTION OF THE DRAWING
Further objects and advantages of the invention will be apparent
from a reading of the following description of the preferred
embodiment thereof in conjunction with the single FIGURE of the
drawing which depicts a block diagram of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawing, a television receiver (not shown)
preferably has its various functions such as channel selection,
on-off and volume change, controlled from a remote position by
means of a hand-held control unit (not shown). Such control units
commonly use different frequency infrared (IR) command signals for
initiating the desired functions. An IR decode unit 11 is shown for
receiving transmitted IR command signals and effecting a desired
function in a control unit 10. Control unit 10 may include a number
of functions as mentioned above as well as include appropriate
tuning circuitry for television signals. It specifically includes
three outputs, one marked V (volume), another marked P (privacy)
and a third marked T (telephone). While the control unit will
usually have additional outputs, only the three mentioned ones are
of interest to this invention.
The T output of control unit 10 is connected to a switch 23 and to
a telephone interface circuit 20 which includes a conventional
telephone induction coil 21 and suitable means such as a hook
switch relay (not shown), for disconnecting the telephone ring
circuit and connecting the telephone talk circuit across the
telephone lines. The ring circuit (not shown) supplies a ring
detector 22, which is responsive to the 20 Hz ringing signal
commonly used in telephone systems. The ring detector is coupled to
an analog to digital (A/D) detector 24 where the 20 Hz ring signal
is "digitized" and supplied to a tone generator 25 and a holding
circuit 26. The output of the holding circuit is supplied to a
digital to analog (D/A) converter 27 where the digitized signal is
converted to an analog signal and supplied to an amplifier 28 and
used to drive a visual indicator 29. The signal indicator may be a
LED (light-emitting diode), for example. The output of a D/A
converter 27 is also used to control switch 58 during an incoming
ring signal.
Switch 23 is connected to another switch 58 in the listen channel.
Switch 58 has two signal inputs, one from the normal television
audio circuit 61 and the other from the output of a D/A converter
56 of the listen channel. The output of tone generator 25, which is
simply a sine wave oscillator, is approximately a 600 Hz signal.
This signal is supplied during an incoming ring signal to the input
of a D/A converter 56 and consequently operation of switch 58
disconnects the normal TV audio signal and connects the tone
generator signal to a volume control 59 in an amplifier which
drives a speaker 60.
The television viewer is alerted to an incoming telephone call by
an audible ring signal from the television speaker and by visual
indicator 29. The viewer "answers" the telephone by sending the
appropriate IR command signal for energizing the T output of
control unit 10 to operate the mechanism in telephone interface
circuit 20. The ringing circuit is switched out and the induction
coil connected across the telephone lines and switch 23 operates,
resulting in switch 58 operating to connect the listen channel to
the audio output circuitry of the television receiver.
The talk channel includes a microphone 30 supplying an amplifier
31, the output of which is coupled to an A/D converter 32 where the
amplified analog microphone signal is digitized, that is, has
discrete numerical values assigned to various amplitude levels. The
output of A/D converter 32 is sent directly to a gain-controlled
amplifier or multiplier 35 having its output supplied to a D/A
converter 36, which reconverts the signal into analog form and
supplies it to induction coil 21 in telephone interface 20. The
output of A/D converter 32 is also supplied to a signal level
detector 33 which in turn has an output supplied to a background
noise cancelling circuit 34. The output of background circuit 34 is
supplied to the input of a variable gain amplifier (multiplier) 40.
While both gain-controlled amplifier 35 and variable gain amplifier
40 are multipliers, use of different names is believed to help in
understanding the description and the claims. The other input to
variable gain amplifier 40, as will be seen, is supplied from the
listen channel. The output of variable gain amplifier 40 is
supplied to an inhibit circuit 41 which is coupled to the P output
of control unit 10. The viewer may energize this ouptut by an
appropriate IR command signal when it is desired to keep sounds in
the room picked up by microphone 30 from being coupled to the
telephone lines.
The output of inhibit circuit 41 is supplied as one input to a
substraction circuit 43, the other input of which comes from the
listen channel. Subtraction circuit 43 develops an output based
upon the larger of its two input signals and supplies an
appropriate switching signal to a switch control generator 44 which
has two outputs A and B. Outputs A and B are selectively energized,
with output A controlling the gain of gain-controlled amplifier 35
in the talk channel and output B controlling the gain of
gain-controlled amplifier 55 in the listen channel. Output A is
also coupled to a scaling circuit 37 for controlling the gain of
variable gain amplifier 42. Similarly output B is coupled to a
scaling circuit 57 for controlling the gain of variable gain
amplifier 40. The scaling circuits are designated 1-A and 1-B which
means that the digital signals on outputs A and B are subtracted
from 1. The digital signals at outputs A and B vary from 0 to
1.
The listen channel is connected to induction coil 21 in the
telephone interface through a DC blocking capacitor 51 which
supplies analog signals to an A/D converter 52 which digitizes
them. The output of the A/D converter is directly supplied to
gain-controlled amplifier 55 which, in turn, supplies D/A converter
56, as explained previously. The output of A/D converter 52 is also
supplied to a signal level detector 53 similar to signal level
detector 33 in the talk channel. The output of detector 53,
however, is supplied to an amplifier 54 prior to being supplied to
an input of variable gain amplifier 42. The output of amplifier 42
is, as noted before, supplied to subtraction circuit 43.
The additional amplifier 54 in the listen channel is included to
increase the sensitivity of the listen channel since it is subject
to many more variables such as signal level, noise, line losses,
etc. and as mentioned previously is considered to be uncontrollable
in comparison with the talk channel.
The magnitude of the signal on output A or on output B of switch
control generator 44 determines the gain of the respective
gain-controlled amplifiers 35 and 55. Switch control generator 44
selectively energizes outputs A and B which are normally at digital
0 indicating that the multipliers to which they are connected are
substantially disabled.
When a signal is received in the talk or in the listen channel, its
associated gain-controlled amplifier is enabled and the signal is
translated through the channel. Since only one of the talk and
listen channels is enabled at any one time, channel control means
are provided for switching the channels as a function of signals
therein. These channel control means include scaling circuits 37
and 57, variable gain amplifiers 40 and 42, differential means, in
the form of subtraction circuits 43, and switch control generator
44.
The channel control means of the invention senses the signal level
in a channel and increases the gain of that channel as a function
of the sensed signal level and offsets cross coupled signals by
desensitizing the other channel. Consequently, rather than trying
to "null out" the inherent cross coupling in the telephone lines,
the invention simply nullifies its effect by offsetting cross
coupling signals. For example, a strong signal in the talk channel
will result in high gain in that channel and a correspondingly
large cross coupled signal in the listen channel. The gain control
factor (A) applied to the gain-controlled amplifier in the talk
channel is scaled and the resultant (1-A) gain factor "multiplied"
with the signal level in the listen channel and applied to the
subtraction circuit. The signal level in the talk channel is
multiplied by the scaled gain control factor of the listen channel
(1-B) which is very close to 1 and applied to the subtraction
circuit. The output of the subtraction circuit determines which
channel is on and at what gain factor.
Since the system is digitized and microprocessor based, it is
extremely predictable and fast-acting with decay times of very
short duration to enable rapid, but controlled, switching.
No adjustments are provided for "tweaking" to a particular
telephone line since the system is independent of the telephone
line parameters, specifically the degree of cross coupling in the
line.
With the system of the invention, the threshold of prior art
circuits is eliminated and cross coupled signals are offset. Weak
signals are offset slightly and strong signals are offset strongly.
Digitizing the signals yields extremely accurate amplitude levels
and predictable results. The very high speed gain control and
switching yields a television receiver two-way communication
channel that approaches the performance of a conventional telephone
system. The system is capable of switching the channels in
mid-syllable.
By way of further detail, signal level detectors 33 and 53 may each
comprise an absolute value detector circuit in series with a peak
detector circuit which, in turn, supplies a decay circuit of
approximately 100 milliseconds. This particular combination has
been found to be well suited to normal speech. Background noise
cancelling circuit 34 may be incorporated for those situations
where relatively constant noise signals are present such as when an
air conditioner is running. This circuit may comprise a very long
time constant filter, on the order of 0.1 Hz, and an override
arrangement to enable quick response in the event that the constant
background noise source is suddenly turned off. One approach would
include a substraction circuit that is supplied with the long time
constant output from the filter and with the output of the 100
millisecond decay circuit in the signal level detector. The output
of the subtraction circuit would be supplied to a comparator along
with the direct output of the 100 millisecond decay circuit with
the comparator output constituting the output of the background
noise cancellation circuit. Such an arrangement precludes
background noise from desensitizing the other channel and avoids
the difficulty inherent in using long time constant filters which
are incapable of responding to changes in background noise.
Switch control generator 44 may comprise a parallel arrangement of
a positive peak detector, a negative peak detector and a zero
crossing detector. The peak detectors supply the input of a pair of
"decay to zero" circuits of approximately 100 millisecond duration
and the zero crossing detector supplies a reset for the decay
circuits.
A computer program is attached as an appendix.
What has been described is a television receiver with improved
two-way telephone communication capability. It is recognized that
numerous modifications and changes in the described embodiment of
the invention will be apparent to those skilled in the art without
departing from the true spirit and scope of the invention as
defined in the claims.
* * * * *